This disclosure relates to an improved device which allows a sodium sulfur cell to become more fully charged and to a method of constructing same.
It has been observed that as a sodium sulfur cell becomes charged there develops a region where two phases are present in the sulfur electrode, namely sodium polysulfide and sulfur. As the charge continues, the sulfur preferentially forms around the beta electrolyte tube. Since the sulfur is an insulator, greater and greater voltages become necessary in order to charge the cell whereupon a point is soon reached as to be harmful to the cell. This reaction may occur immediately following the initial formation of the sulfur.
It is known that this preferential formation of sulfur around the beta electrolyte tube may be considerably delayed by placing a more resistive layer of current collector, than is in the remainder of the positive (sulfur) electrode, about the beta electrolyte tube. This layer may be of a material which is wetted by both sulfur and sodium polysulfide. Also, the material must be inert to the cell atmosphere. A type of graphite mat has previously been employed for this purpose.
However, certain problems exist in the use of the graphite mat. Initially, the mat consists of very delicate fibers of graphite which are prepared to a size of 7cm .times. 2cm weighing 0.05 grams. The weight is very critical to cell performance. Heavier mats placed into cells give high resistance to the cell while lower weights do not allow the cell to be charged into the two-phase region thereof. In addition, the method of preparing such mats presently includes cutting to a 7cm .times. 2cm size with a razor blade and then peeling off laminates of the fibrous mat. In conducting this operation a person's individual judgment is employed as to the proper thickness of the laminate and then the pieces are weighed with the hope that the weight will be correct.
Since the portions of the laminate are very delicate, if the pieces are underweight they can not be brought up to the correct weight by adding more mat material. If they are overweight, it is very difficult to peel off any more graphite from the mat since it is structurally very weak. Therefore, it is evident that there is a significant difficulty in working with the delicate mat material. In any large scale production of these devices the problems become greater as the mat is inconsistent in its structure and will therefore present a varying resistance in its cross-section. Its delicate physical makeup presents severe handling problems.
Fiberglass sleeves have been attempted to be used in cells of the kind described, but it has been observed that the silica which is present can be harmful to the cell. Furthermore, tests have shown that the original graphite mat gives superior performance compared to the fiberglass sleeves.